Phase modulation of a data signal to be transmitted may be BPSK (binary phase shift keying) or QPSK (quadrature phase shift keying), or OQPSK (offset quadrature phase shift keying) digital modulation. In the first case, BPSK digital modulation is defined with two phase values or states with a phase shift of 180° between the two phase states. In the second case, QPSK digital modulation is defined with four phase values or states with a phase shift of 90° between each phase state. This modulation in the transmitter usually uses two quadrature carrier signals phase-shifted by 90° from each other before frequency conversion for the transmission of a QPSK modulated signal. In the third case, OQPSK digital modulation is similar to QPSK digital modulation, but can be more advantageous in the event of amplifier non-linearities in the modulation chain.
By way of illustration, this type of QPSK modulation signal is represented in the time domain in FIG. 1. FIG. 1 shows the 2-bit encoding by phase shift keying of an in-phase data signal I and of a quadrature data signal Q phase-shifted by 90° from in-phase data signal I. For transmission of a phase modulation signal, data signals I and Q are added together. The data flow is defined by 1/Ts, where Ts is the duration of an encoding state.
Demodulation of this type of digital phase modulation signal can be performed synchronously in a phase modulation signal receiver. Demodulation can generally take place after at least a first frequency conversion of the phase modulation signal captured by an antenna of the receiver. To enable phase demodulation in synchronous mode, it is necessary to recover the carrier frequency of an intermediate signal, or of a signal directly captured by the antenna.
Recovery of the carrier frequency makes it possible to extract the modulating signal. To achieve this, it is known to use a Costas loop to recover the carrier frequency, in order to extract the modulating signal. Demodulation of a phase modulation signal is also explained in the article entitled “PSK Demodulation (Part I)” by J. Mark Steber in The Communications Edge by WJ Communications, Inc, revised in 2001.
FIG. 2 shows a diagram or constellation of the phase states of a QPSK modulation signal. The phase states are shown in polar coordinates relative to a real axis for in-phase data signal I and an imaginary axis for quadrature data signal Q. If the carrier frequency of at least one conversion signal of the electronic circuit is not equal to the carrier frequency of the phase modulation signal to be demodulated, there remains a phase error φ. This carrier frequency must be recovered exactly in the electronic circuit in order to be able to demodulate the phase modulation signal in a control loop, and to correct the phase as explained hereafter. After recovering the carrier frequency, the modulating signal can be extracted, which corresponds to points 11, 10, 00 or 01 shown in FIG. 2.
In a known electronic demodulation circuit for a phase modulation signal as shown in FIG. 3, the intermediate frequency signal IF is provided both to a first signal mixer 2, and to a second signal mixer 3. Frequency conversion of the intermediate frequency signal IF is performed in the first and second mixers 2, 3 with oscillating signals. A first non-phase-shifted oscillating signal, which originates from a voltage controlled oscillator (VCO) 8 is provided to first mixer 2, and a second oscillating signal, which is phase-shifted by 90° in a phase shifter 9 and which originates from VCO oscillator 8, is provided to second mixer 3. The output of first mixer 2 provides an in-phase baseband signal I, while the output of second mixer 3 provides a quadrature baseband signal Q. Filtering is performed on the in-phase and quadrature signals by two low-pass filters 4 and 5 to provide an in-phase data signal IOUT and a quadrature data signal QOUT.
If the phase and frequency of the oscillating signals are not quite equal to the phase and frequency of the carrier of intermediate signal IF, there remains a frequency and phase error. A phase comparator 6 is thus used to compare in-phase data signal IOUT and quadrature signal QOUT. The phase error is provided through a loop filter 7, which is a standard filter such as an integrator, to the input of voltage controlled oscillator 8 in a Costas loop. Analogue or digital embodiment of the Costas loop requires low-pass filters, which have the disadvantage of becoming cumbersome, when their cut-off frequency is low.